The invention relates to an alignment device for aligning a first object, which is provided with at least a first alignment mark, with respect to a second object which is provided with at least a second alignment mark, said device comprising a radiation source for supplying at least an alignment beam, a first object holder, a second object holder, an imaging system for imaging said first alignment mark and said second alignment mark onto each other, and a radiation-sensitive detection system arranged in the path of selected alignment beam portions coming from a first alignment mark and a second alignment mark onto which the first alignment mark is imaged, the output signal of the detection system being indicative of the extent to which the first and the second object are aligned with respect to each other.
The invention also relates to a lithographic apparatus for repetitively imaging a mask pattern on a substrate, in which apparatus such an alignment device for aligning a mask with respect to a substrate is used. The mask has at least a mask alignment mark and the substrate has at least a substrate alignment mark.
Aligning, for example, a mask alignment mark and a substrate alignment mark with respect to each other is understood to mean both directly and indirectly aligning these alignment marks. In the case of direct alignment, a substrate alignment mark is imaged on a mask alignment mark, or conversely, and the detection system is arranged behind the last mark. In the case of indirect alignment, both the substrate alignment mark and the mask alignment mark are imaged on different parts of a further, reference, mark, and the detection system is arranged behind the reference mark. In the latter case, the extent to which the substrate alignment mark and the mask alignment mark are aligned with respect to each other is determined by detecting to what extent both the substrate alignment mark and the mask alignment mark are aligned with respect to the reference mark.
The selected alignment beam portions are those portions of the alignment beam which are effectively used to image the first alignment mark on the second alignment mark. If the alignment marks are diffraction gratings, the selected alignment beam portions are the beam portions diffracted in given orders, for example the first orders, by the alignment marks.
U.S. Pat. No. 4,778,275 describes an optical lithographic projection apparatus for repetitive and reduced imaging of a mask pattern, for example, the pattern of an integrated circuit (IC) on a number of IC areas, or substrate fields, of the substrate. The mask and the substrate are moved with respect to each other between two successive illuminations, for example, along two mutually perpendicular directions in a plane parallel to the substrate plane and the mask plane so as to successively image the mask pattern in all substrate fields.
Integrated circuits are manufactured by means of diffusion and masking techniques. A number of different mask patterns are consecutively imaged on one and the same location on a semiconductor substrate. Between the consecutive imaging steps on the same locations, the substrate must undergo the desired physical and chemical changes. To this end, the substrate must be removed from the apparatus after it has been illuminated with a first mask pattern and, after it has undergone the desired process steps, it must be placed in the apparatus again in the same position so as to illuminate it with a second mask pattern, and so forth. It must then be ensured that the projections of the second mask pattern and of the subsequent mask patterns are positioned accurately with respect to the substrate.
The lithographic techniques can also be used in the manufacture of other structures having detail dimensions of the order of micrometers or less, such as structures of integrated planar optical systems, magnetic heads, or structures of liquid crystalline display panels. Also in the manufacture of these structures, images of the mask pattern must be aligned very accurately with respect to a substrate.
In order to be able to realize the desired, great positioning accuracy, within several tenths of one micrometer in the apparatus according to U.S. Pat. No. 4,778,275, of the projection of the mask pattern with respect to the substrate, this apparatus comprises a device for aligning the substrate with respect to the mask pattern with which an alignment mark provided in the substrate is imaged on an alignment mark provided in the mask. If the image of the substrate alignment mark accurately coincides with the mask alignment mark, the substrate is correctly aligned with respect to the mask pattern. In the known alignment device, a HeNe laser beam is used as an alignment beam.
In connection with the increasing number of electronic components per IC and the resultant smaller dimensions of these components, increasingly stricter requirements are imposed on the accuracy with which ICs can be manufactured. This means that a mask pattern must be aligned with respect to the substrate fields with an increasing accuracy.
The alignment device described in U.S. Pat. No. 4,778,275 has hitherto worked to full satisfaction, but it is to be expected that with decreasing detail sizes, or line widths, of the IC patterns and with the use of novel technologies in IC manufacture, the alignment device may present problems relating to its reliability and accuracy. These problems relate to the symmetry of an alignment mark, required for alignment, and the symmetry of the processes to which the substrate, hence also the substrate alignment mark, is subjected. The known alignment device is only reliable if both the alignment mark and the processes are symmetrical.
In the manufacture of new-generation ICs with smaller line widths, the resolving power of the projection lens system used for the mask pattern projection must be increased, which means that the numerical aperture of this system must be increased. This means that the depth of focus of this system decreases. Since there will be some curvature of the image field at the desired relatively large image field of the projection lens system, there is substantially no tolerance for the evenness of the substrate. To maintain the desired evenness of the substrate, it may be polished, in between to illuminations of the substrate, by means of the chemical mechanical polishing (CMP) process. This polishing process is found to cause an asymmetrical distortion in a substrate alignment mark implemented as a grating. Besides the CMP process, the manufacture of ICs has also become more and more complex by the use of non-uniform etching processes and the provision of an increasing number of metal layers on the substrate. This also leads to an asymmetrical distortion of the substrate alignment mark. Moreover, these substrates, and hence the alignment marks, are coated with a number of transparent layers, such as oxide layers, nitride layers and poly layers. These layers may be deposited isotropically, but they may cause interference effects in the alignment beam affecting the alignment. Particularly the combination of asymmetry in the alignment mark and interference effects may give rise to relatively large alignment errors.
It is an object of the present invention to provide an alignment device in which the above-mentioned problems do not occur and which is more accurate and more reliable than the known devices. This alignment device is characterized in that the wavelength of the alignment beam is of the order of at least 1000 nm and at most 1100 nm.
The invention is based on the recognition that, taking into account the relevant parameters of the period of the grating mark, the chosen diffraction orders, the profile of the mark including the asymmetrical shape and the depth, the deposited dielectric layers and metal layers and the standard deviation of the alignment error, a more accurate and reliable alignment error detection is possible as the wavelength of the alignment beam is larger. The upper limit for this wavelength is limited by the material of the substrate; for a wavelength of more than 1100 nm, silicon becomes transparent to the alignment beam and its rear side is visible to the alignment device. The standard deviation is the deviation which results from a variation of the parameter: layer thickness or depth of the mark. The fact that a choice has been made for this standard deviation means that the criterion is not the average value of zero but that the position of the substrate alignment mark, observed by the device, must be constant. The requirement is that the observed position of the mark remains constant, for example, at varying layer thicknesses, even if the mark is asymmetrical and is independent of the thickness of the radiation-sensitive layer on the substrate, or the layer packet deposited on the mark.
It is to be noted that the English-language abstract of JP-A 63-40316 states that interferences in the radiation-sensitive layer result in an arbitrary variation of the intensity of the reflected alignment beam, so that the diffraction pattern becomes indefinite. However, in this abstract it is proposed to vary the wavelength of the read beam. This solution has the same drawback as the alternative solution of using a broadband alignment beam, namely the elements of the alignment device must be made suitable for broadband radiation, which complicates this device to a considerable extent. Moreover, a weak detection signal would then be obtained and no order diaphragm for selecting suitable diffraction orders could be placed in the alignment beam.
A preferred embodiment of the alignment device is further characterized in that the radiation source is constituted by one of the following lasers:
an Nd:YAG laser having a wavelength of 1064 nm;
an Nd:YLF laser having a wavelength of 1047 nm;
a semiconductor laser having a wavelength of 980 nm.
These lasers are already manufactured in large numbers for other applications and are very suitable for use in the novel alignment device.
The alignment device may be further characterized in that the detection system comprises an InGaAs detector.
This detector has the desired sensitivity to said wavelengths.
It is also possible to use a Si-detector.
The invention also relates to a lithographic apparatus for imaging a mask pattern on a substrate, which apparatus comprises an illumination unit for illuminating a mask with a projection beam, a mask holder, a substrate holder, and a projection system arranged between the mask holder and the substrate holder, and further comprises a device for aligning the mask and the substrate with respect to each other. This apparatus is characterized in that the alignment device is constituted by the alignment device described hereinbefore, wherein the substrate and the mask constitute the first and the second object for the alignment device.
The most customary embodiment of this apparatus is further characterized in that the projection beam is a beam of electromagnetic radiation, and the projection system is an optical projection system, and in that the imaging system of the alignment device also comprises the optical projection lens system.
However, the projection beam may also be a charged-particle beam such as an ion beam, an electron beam or a beam of X-ray radiation, for which the projection system is adapted to the type of radiation. For example, if the projection beam is an electron beam, the projection system will be an electron lens system. This projection system then no longer forms part of the imaging system of the alignment device.
At the above-mentioned wavelengths for the alignment beam, imaging errors, namely a magnification error and a focusing error, are produced when using the projection system for imaging a first alignment mark on a second alignment mark. The reason is that the projection system is optimized for the short-wave projection radiation, for example deep UV radiation. The difference between the wavelengths of the projection radiation and that of the alignment radiation is now even larger than in known apparatuses, in which the alignment radiation has a wavelength of 633 nm.
To prevent these imaging errors, the apparatus according to the invention is further characterized in that a correction element for correcting the direction and convergence of the alignment beam portion reflected by an alignment mark is arranged between the substrate holder and the mask holder, said correction element having a dimension which is considerably smaller than the diameter of the projection beam in the plane of the correction element.
The principle of using such a correction element in an alignment device is described in U.S. Pat. No. 5,100,237. This patent also gives details about the position of the correction element, and some embodiments of this element are mentioned. In an alignment device with the proposed wavelength for the alignment beam, the correction element is even more needed than in the apparatus described in U.S. Pat. No. 5,100,237.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.